Research Article
www.acsami.org
Low-Cost Al2O3 Coating Layer As a Preformed SEI on Natural
Graphite Powder To Improve Coulombic Efficiency and High-Rate
Cycling Stability of Lithium-Ion Batteries
Tianyu Feng,†,‡,§ Youlong Xu,*,†,‡ Zhengwei Zhang,† Xianfeng Du,†,‡ Xiaofei Sun,†,‡ Lilong Xiong,†,‡
Raul Rodriguez,∥ and Rudolf Holze*,§
†
Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric
Research, Xi’an Jiaotong University, Xi’an 710049, China
‡
Shaanxi Engineering Research Center of Advanced Energy Materials & Devices, Xi’an Jiaotong University, Xi’an 710049, China
§
Institut für Chemie, AG Elektrochemie, Technische Universität Chemnitz, 09111 Chemnitz, Germany
∥
Institut für Physik, Technische Universität Chemnitz, 09111 Chemnitz, Germany
S Supporting Information
*
ABSTRACT: Coulombic efficiency especially in the first
cycle, cycling stability, and high-rate performance are crucial
factors for commercial Li-ion batteries (LIBs). To improve
them, in this work, Al2O3-coated natural graphite powder was
obtained through a low-cost and facile sol−gel method. Based
on a comparison of various coated amounts, 0.5 mol %
Al(NO3)3 (vs mole of graphite) could bring about a smooth
Al2O3 coating layer with proper thickness, which could act as a
preformed solid electrolyte interface (SEI) to reduce the
regeneration of SEI and lithium-ions consumption during
subsequent cycling. Furthermore, we examined the advantages
of Al2O3 coating by relating energy levels in LIBs using density
functional theory calculations. Owing to its proper bandgap and lithium-ion conduction ability, the coating layer performs the
same function as a SEI does, preventing an electron from getting to the outer electrode surface and allowing lithium-ion
transport. Therefore, as a preformed SEI, the Al2O3 coating layer reduces extra cathode consumption observed in commercial
LIBs.
KEYWORDS: lithium-ion batteries, anodes, alumina coating, SEI, DFT calculations, energy levels
1. INTRODUCTION
Commercial Li-ion batteries (LIBs) use a graphite-based carbon
anode. During the initial lithiation cycles, a solid electrolyte
interphase (SEI) forms on the graphite anode surface due to
electrochemical instability of the electrolyte versus lithiated
graphite.1−7 Ideally, the SEI inhibits further reduction of the
electrolyte, allows Li-ion conduction, and is electronically
insulating. However, in fact, the fragile and nonuniform SEI
film will crack caused by surface defects and anisotropic rough
edges and will be re-formed again and again during charging
and discharging. This will consume the limited supply of Li ions
continuously in a full battery and therefore result in capacity
fading.7
Much effort has been devoted to overcome this problem, and
one effective solution is the surface modification of anodes by
methods such as mild oxidation8,9 and metal or medal oxide
coating.10−12 He et al. deposited alumina on an etched silicon
surface by atomic layer deposition (ALD) and obtained a great
improvement of capacity retention and Coulombic efficiency
(CE).10 His group also found that the open-circuit voltage for
© 2016 American Chemical Society
the anode coating with alumina was lower than that without
coating, showing that alumina can reduce side reactions
between electrode and electrolyte.10 Riley et al. improved
electrochemical performance of MoO3 nanoparticles as highcapacity/high-volume expansion anodes for Li-ion batteries by
alumina surface coating using ALD.13 Jung et al. performed
ALD alumina coating on natural graphite (NG) and LiCoO2 to
enhance their long-term cycling performance.14,15 For the
mechanism, most of the above concluded the protection effect
for electrode by alumina coating. Kim et al. pointed out that
amphoteric oxide alumina can protect the electrode surface
from acid attack by scavenging HF and H2O from the
electrolyte.16 Jung and Han found that, after the lithiation of
the Al2O3 coating layer reached a thermodynamically stable
phase, extra Li atoms overflowed into the electrode by passing
through the coating layer.17 It was reported that Al2O3 coating
Received: January 7, 2016
Accepted: February 25, 2016
Published: February 25, 2016
6512
DOI: 10.1021/acsami.6b00231
ACS Appl. Mater. Interfaces 2016, 8, 6512−6519
Research Article
ACS Applied Materials & Interfaces
Table 1. Sample Treatments and Names, CE, and Specific Capacity in First Cycle at Current Density of 15 mA/ga
sample
G0
G5A
G05A
G05U
G005A
G005U
a
solution A
5 mol % Al(NO3)3
0.5 mol % Al(NO3)3
0.5 mol % Al(NO3)3
0.05 mol % Al(NO3)3
0.05 mol % Al(NO3)3
solution B
first CE, %
first specific charge capacity, mA h/g
first Sspecific discharge capacity, mA h/g
5 mol % ammonia
0.5 mol % ammonia
0.5 mol % urea
0.05 mol % ammonia
0.05 mol % urea
85.10
81.30
88.80
90.70
89.20
90.0
320.2
239.6
289.3
320.4
312.4
320.5
376.4
294.9
325.6
353.3
350.3
356.1
The molar ratios of Al(NO3)3, ammonia, and urea refer to the graphite content.
black (3 wt %) was prepared by stirring for 180 min with a proper
amount of NMP. The slurry was coated onto a copper foil to a
thickness of 200 μm using the doctor-blade technique. The electrode
was placed in a vacuum oven at 120 °C overnight to dry the electrode
and stored in an Ar-atmosphere glovebox.
CR2016 coin cells were fabricated to study the electrochemical
performance. The rolled electrode film was cut into 2 cm2 circular
discs as the anodes. The counter electrode was lithium metal foil. The
electrolyte solution was 1 M LiPF6 in EC (ethylene carbonate):EMC
(ethyl methyl carbonate) (1:1 by volume). Coin cells were assembled
in an argon-filled glovebox (Mikrouna Super (1225/750)) with H2O
and O2 concentrations below 0.1 ppm. Electrochemical performance
of the cells was evaluated by galvanostatic discharge/charge measurement between 0.01 and 3 V using a computer controlled battery tester
(LANHE CT2001A, Wuhan, China). The electrochemical impedance
spectra (EIS) were measured at 0.01, 0.1, and 0.5 V in the fifth cycle
with a sine voltage of 5 mV in the frequency range from 100 kHz to 10
mHz using a versatile multichannel galvanostat 2/Z (VMP2, Princeton
Applied Research, Oak Ridge, TN, USA).
2.3. DFT Calculations. For all solvent molecules, Kohn−Sham
DFT calculations were performed using Dmol3 code.21,22 The
generalized-gradient approximation (GGA) with the Perdew−
Burke−Ernzerhof functional (PBE)23 and Perdew−Wang generalized-gradient approximation (PW91)24 was used to describe the
exchange-correlation effects. The descriptions of the valence states
were obtained with the double numerical basis set augmented with
polarization p-function (DNP)21 which has a computational precision
being comparable with the Gaussian split-valence basis set 6-31G**.
Global orbital cutoff was 3.7 Å for all the molecules. Spin-unrestricted
wave functions were used for each molecule with charge of 0, −1, and
+1, respectively. The energy convergence for geometry optimization
was set to 1 × 10−5 Ha. A Gaussian smearing of 0.005 Ha was used to
achieve self-consistent field convergence.
For all bulk materials, calculations were performed using CASTEP
code, which adopts fully self-consistent DFT calculations to solve
Kohn−Sham equations. For graphite, van der Waals (vdW)
interactions were taken into consideration using the Tkatchenko−
Scheffler (TS) scheme25 of dispersion correction DFT (DFT-D). The
GGA, with the functional PBE23 and PW9124 was employed. The
electronic wave functions were expanded as a linear combination of
plane waves, with a kinetic energy cutoff of 500 eV. The ultrasoft
pseudopotentials for Li, C, Al, and O were used in all calculations. We
used a method of Gaussian smearing to achieve self-consistent field
convergence with a smearing value of 0.1 eV. The energy and force
convergence tolerance was 5.0 × 10−6 eV/atom and 0.01 eV/Å,
respectively. A five to ten atomic layer slab with a vacuum region of 35
Å in the vertical direction was employed as the surface model
framework for bulk materials. All atoms were relaxed into their ground
states by a conjugate-gradient algorithm except the middle layers
which were kept frozen mimicking bulk materials.
may act as a solid electrolyte and may prevent direct contact
between the cathode surface and the electrolyte.15,18,19 Based
on first-principles calculations, Hao et al. suggested that in bulk
Al2O3 the Li diffusivity is low, while in coating materials other
factors, such as grain boundary and short length scale of
coatings, might provide only a small resistance for Li-ion
transport.20 However, (i) ALD is very costly and complex and
is not suitable for mass production. (ii) The reason for the
superiority of alumina coating was not investigated with respect
to energy levels.
Herein, we demonstrate a low-cost treatment to modify NG
by coating with Al2O3 based on a sol−gel method to improve
its CE (especially the CE in the first cycle), cycling stability, and
rate performance. From a fundamental point of view, we
examined the advantages of alumina coating with respect to
relative energy levels in Li-ion batteries using first-principles
density functional theory (DFT) calculations.
2. EXPERIMENTAL SECTION
The natural graphite powder YAG-1 used in this study was obtained
from UNION AMPEREX Co. (Henan Province, People’s Republic of
China). The particle size D50 and Brunauer−Emmett−Teller (BET)
surface area of the natural graphite are 18−24 μm and 5.0 m2/g,
respectively.
2.1. Preparation. A 0.6 g amount of YAG-1 graphite particles
(G0) was added into 50 mL of aqueous solution with various molar
concentrations of Al(NO3)3 (vs graphite) (solution A). After 30 min
of stirring, 50 mL of ammonia solution or urea solution with the same
molar concentration of Al(NO3)3 (solution B) was added dropwise to
solution A under vigorous stirring. The final solution was dried and
concentrated at 70 °C on a water bath under stirring resulting in a gel.
Subsequently the gel was transferred into a tube furnace and heated in
argon atmosphere at 900 °C for 6 h. The treatments and names for
each sample are listed in Table 1.
2.2. Characterization. The ζ potential of the graphite powder and
the sol solution were measured with a Zetasizer NanoZS 90 (Malvern
Instruments Ltd., Malvern, U.K.) with a measure angle of 90° and a
laser wavelength of 633 nm at 25 °C. A suspension of 5 mg of graphite
powder in 20 mL of DI water was used for graphite ζ potential
characterization. A mixture of 10 mL of 0.2 M Al(NO3)3 and 10 mL of
0.2 M NH3·H2O or urea was used for colloidal ζ potential
characterization. Field emission scanning electron microscopy (FESEM, Quanta 250FEG, FEI, Hillsboro, OR, USA) and transmission
electron microscopy (TEM, JEM-2100, JEOL Ltd., Tokyo, Japan)
were employed to investigate the surface morphology of graphite
samples. An energy dispersive spectrometer (EDS, EDAX TEAM
Apollo XL-SDD, Oxford Instrument) attached to the SEM was used to
analyze elements on the surface. The crystalline structures were
characterized by powder X-ray diffraction (XRD) measurements using
an X’Pert PRO (PANalytical Ltd., Eindhoven, Holland) diffractometer
equipped with Cu Ka radiation by step scanning in the 2θ range of
10−80°.
Graphite electrodes were prepared using polyvinylidene fluoride
(PVDF) as binder and N-methylpyrrolidinone (NMP) as solvent. A
mixture of graphite powder (92 wt %), PVDF (5 wt %), and carbon
3. RESULTS AND DISCUSSION
Figure 1 shows schematic relative energy levels in a typical Liion battery. The open-circuit voltage, UOC, of a cell is the
difference between the electrochemical potentials μA and μC of
the anode and cathode:
6513
DOI: 10.1021/acsami.6b00231
ACS Appl. Mater. Interfaces 2016, 8, 6512−6519
Research Article
ACS Applied Materials & Interfaces
With Al2O3 coating, probably because of its proper bandgap
and lithium-ions conduction ability,17,20,26 the coating layer
performs a function similar to that of an SEI, preventing
electron transfer and allowing lithium-ion transport. As a
preformed SEI, the Al2O3 coating layer decreases the lithiumion consumption for generation of SEI. Therefore, it can
improve CE and reduce required inventory of lithium ions in
commercial batteries.
ζ potential is the electric potential in the interfacial double
layer (DL) at the location of the slipping plane relative to a
point in the bulk fluid away from the interface. It has been
widely used to characterize the interactions between colloids or
particles. Figure 2a shows the ζ potential of the graphite
particle G0 in water and the colloids of 0.1 M [Al(NO3)3 +
NH3·H2O] and 0.1 M [Al(NO3)3 + urea], respectively. The
results are consistent with previously reported ones.27−30 They
suggest that colloids with positive charge will attach onto
graphite particles with negative charge by electrostatic
attraction spontaneously, as shown in Figure 2b, suggesting
that surfactants are not necessary for this sol−gel coating
method.
In Figure 3, the samples with different coating amounts were
investigated by FE-SEM analysis. The rough surface of pristine
NG powders with many flakes would result in coarse and much
SEI formation, consuming a lot of lithium ions while cycling. In
the coating process, the ammonia added into the solution
increased its pH so as to form Al(OH)3 sol to enhance the
adhesion of Al3+ on graphite particles. With thick coating, as
shown in Figure 3b, the cracks on the surface of G5A were
harmful to formation of a smooth SEI, indicating that the
coating was too thick. Cracks may be caused by the different
strains of graphite and the coating layer. Its specific charge/
discharge capacity was lower (see below) than that of others,
which could also be attributed to the thick coating. G05A and
G05U (Figure 3e,f) showed a relatively smooth surface which
could act as a preformed SEI.
To further improve the coating method, urea was used
instead of ammonia for the following reasons. First, an aqueous
solution of urea has a lower pH than that of ammonia at the
same concentration so that the initial colloidal particles will be
much smaller and more uniform. In addition, urea will
decompose gradually when heated to over 60 °C. The colloidal
particles attached onto the graphite surface would grow larger
and merge into a uniform layer finally (G05U in Figure 3e,f).
Moreover, Figure 3g shows the relative atomic composition of
the area of G05U in Figure 3f by EDS. In addition, as shown in
Figure 3i,j,k,l, with 0.05 mol % coating, G005A and G005U had
thinner coating layers.
TEM was performed to study the morphology and lattice
plane of G05U, as shown in Figure 4. To analyze the crystal
structure clearly, FFT images for various areas were aquired to
study the crystal plane from outside in. The lattice plane for
each pattern was pointed out correspondingly, according to its
symmetrical length. The patterns of Al2O3 (113) and graphite
(002) indicated their good crystallinity and polycrystallinity of
Al2O3.
Figure 5 presents the XRD patterns of the pristine and the
Al2O3-coated NG samples. A logarithmic y-axis is used to
highlight the peaks of Al2O3 in the presence of the extremely
high peak of graphite (002). The intensities of Al2O3 peaks in
G05A and G05U were higher than those in G005A and
G005U, indicating they contained a greater amount of Al2O3,
which is consistent with their SEM images in Figure 1.
Figure 1. Schematic energy levels in a typical Li-ion battery. The
LUMO of the electrolyte is lower than the Fermi energy of Li (ELi),
the difference between which is noted as ΔEred. EIE stands for
ionization energies of various organic solvent molecules.
UOC = (μA − μC )/e
(1)
Theoretically, μA and μC should be both between LUMO and
HOMO of the components of the electrolyte solution so that
electrolyte will not decompose. But, in fact, μA above the
electrolyte LUMO reduces the electrolyte unless the anode−
electrolyte reaction becomes blocked by the formation of a
passivating SEI layer; similarly, a μC located below the HOMO
oxidizes the electrolyte unless the reaction is blocked by an SEI
layer.7
DFT calculations were used to supplement the insufficient
experimental data (see Table 2). In Supplementary Information
Table 2. ΔEred for Various Electrolyte Solvents by DFT
Functionals of PBE and PW91, Respectively
ΔEred (eV)
VC (vinylene carbonate)
FEC (fluoroethylene carbonate)
EC (ethylene carbonate)
DMC (dimethyl carbonate)
DEC (diethyl carbonate)
PC (propylene carbonate)
MeOH (methanol)
H2O
PBE
PW91
1.414
1.392
0.927
0.677
0.194
0.718
0.582
2.194
1.464
1.443
0.989
0.726
0.247
0.773
0.598
2.273
(SI) Table S1 and Table S2, details of the LUMO−HOMO
gaps and ionization energies (EIE) of various organic solvent
molecules and the work function φ of bulk Li, Al2O3, and
graphite are collected. Experimental data are compared with
calculated ones. The LUMO of the electrolyte is lower than the
Fermi energy of Li (Ef,Li), the difference between which is
noted as ΔEred. This is consistent with the view of Goodenough
and Park that Ef,Li ∼ 1.1 eV above the LUMO of the common
DMC/DEC electrolyte solutions.7 As the graphite anode
becomes lithiated, Ef,graphite will shift up gradually to the end
of the charging voltage (in our work, the end of lithiation
potential was set to 0.01 V vs Li/Li+). Since the charge/
discharge plateau of graphite is ∼0.1 V vs Li/Li+, μA of lithiated
graphite is ∼0.1 eV lower than Ef,Li. This suggests that the
decomposition of electrolyte solution will still take place
yielding a SEI film on graphite anodes.
6514
DOI: 10.1021/acsami.6b00231
ACS Appl. Mater. Interfaces 2016, 8, 6512−6519
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a) ζ potential of G0, 0.1 M [Al(NO3)3 + NH3·H2O] and 0.1 M [Al(NO3)3 + urea], respectively. (b) Scheme depicting adhesion of colloid
onto graphite powder due to electrostatic attraction.
Figure 3. SEM/EDS analysis of pristine and Al2O3-coated graphite. (a) Pristine natural graphite powder G0. (b) G5A graphite powder. The dotted
red rectangular shows a crack in the coating on the powder. (c, d) G05A graphite powder. (e, f) G05U graphite powder. (i, j) G005A graphite
powder. (k, l) G005U graphite powder. The dotted red rectangular in (c, e, i, k) shows the area of magnification. The EDS area scanning result for
G05U powder is shown in g. (h) Schematic of the coating.
and the hexagonal structure with space group P63/mmc,
respectively.
As shown in Figure 6, the open-circuit voltage (OCV) for
each sample was measured 24 h later after cell assembly for
relaxation before any electrochemical tests. The cell with
pristine graphite powder G0 showed higher OCV than any
Accordingly, the smaller peaks of Al2O3 for G005A and G005U
suggested their thinner coatings. Besides, all of the NG peaks
did not show any evident changes, revealing that the coating
treatment did not influence the crystal structure of core
material. All diffraction peaks of Al2O3 and graphite can be
indexed to the rhombohedral structure with space group R3̅c
6515
DOI: 10.1021/acsami.6b00231
ACS Appl. Mater. Interfaces 2016, 8, 6512−6519
Research Article
ACS Applied Materials & Interfaces
Figure 6. Comparison of the OCVs measured 24 h later after cell
assembly for relaxation before any electrochemical tests.
layer effectively prevented some side reactions between the
electrode and the electrolyte solution. It also implies that G05U
with the lowest OCV had the most uniform and effective
coating.
Theoretical specific capacity of graphite is 372 mA h/g
assuming a lithiated state LiC6 with a charge/discharge
potential plateau of ∼0.1 V vs Li/Li+.31 The CE, specific
charge, and discharge capacity in the first cycle for the samples
with various treatments were summarized in Table 1.
Commonly during lithiation of anode materials in the first
cycle, most lithium ions are consumed to form the SEI layer on
the surface. Therefore, the CE in the first cycle is more
indicative than that in following cycles.
Figure 7 shows initial charge/discharge profiles of the
samples at a current density of 15 mA/g in the electrochemical
Figure 4. TEM images of G05U powder. Panels a1−a4 show FFT
images of the red rectangular areas in panel a, respectively. In a2−a4,
crystal faces and XRD reference card numbers are shown.
Figure 5. XRD patterns of G0, G05A, G05U, G005A, and G005U (a).
The peaks around 36° and 67° corresponding to Al2O3 (104), (110),
(214), and (300) are magnified and shown in insets b and c,
respectively. The specific peaks of Al2O3 (113), (024), and (116) are
also labeled in panel a. The reference patterns for Al2O3 and graphite
are presented at the bottom of panel a.
other cells with Al2O3 coating anodes. G05U presented the
lowest OCV. According to energy conservation during reaction
(eq 2), higher U means higher ΔG, i.e., more “reactivity”.
(2)
Figure 7. Initial voltage profiles of the samples obtained at 15 mA/g in
the electrochemical window of 0.01−3 V vs Li/Li+.
where ΔG is the Gibbs free energy change, n is the number of
electrons transferred, U is the cell voltage, and F is the Faraday
constant. There is a negative sign because a spontaneous
reaction has a negative ΔG and a positive U.
This indicates that the interaction between G05U and the
electrolyte was the weakest while that of G0 was the strongest.
This agrees well with the report by He et al. that patterned
silicon electrodes without alumina coating were more reactive
toward the electrolyte solution than coated ones.10 The Al2O3
window of 0.01−3 V vs Li/Li+. The voltage step at ∼1.2 V
versus Li/Li+ corresponding to side reactions including SEI
formation32 was reduced with coated NG, which suggests that
the coating method decreased the irreversible capacity loss for
the electrodes. The much lower specific charge capacity of G5A
than that of G0 might result from its thick coating, which can
be seen in Figure 3b. G05U showed the smallest discharge
plateau between 1.3 and 0.1 V and a relatively high specific
ΔG = −nFU
6516
DOI: 10.1021/acsami.6b00231
ACS Appl. Mater. Interfaces 2016, 8, 6512−6519
Research Article
ACS Applied Materials & Interfaces
charge capacity, suggesting that its coating layer could decrease
the consumption of lithium ions to form SEI and not provide
much resistance for lithium ions to transport. The superiority
performance of G05U could be attributed to its smooth
coating, as shown in Figure 3e,f. Even the thinner coating of
G005A and G005U apparently results in a relatively high first
CE and specific capacity at a low current density of 15 mA/g
(see also Table 1).
To evaluate the impact of Al2O3 coating on rate performance
and cycling stability, the cells were tested at rates varying from
15 to 480 mA/g, as presented in Figure 8. The specific charge
Figure 9. Series of mass-normalized impedance spectra for electrodes
in the fifth cycle at 0.01, 0.1, and 0.5 V, respectively.
Table 3. Most reversible specific capacity and lithiation/
delithiation reactions took place at this potential plateau as
Table 3. Rf and Rct in Each Electrode in the Fifth Cycle at
0.01, 0.1, and 0.5 V, Respectively, Corresponding to the
Curves in Figure 9a
Figure 8. Rate performance of the samples obtained from 15 to 480
mA/g between 0.01 and 3 V vs Li/Li+.
R f ,# Ω
capacity of G005A and G005U is relatively high at low current
density but low at higher current density, which can be
attributed to their thinner coating that might not be able to act
as strong and thick enough preformed SEI layers at high
current density. As the thickness of SEI will increase and
decrease while discharging (lithiation) and charging (delithiation), respectively,32 the thinner coating could not prevent the
formation and deformation of SEI while cycling and resulted in
deterioration of cycling stability at high current density finally.
G05U displayed superior rate performance and cycling stability,
especially at relatively high current density, which could result
from its relatively smooth Al2O3 coating with optimum
thickness. The coating with proper thickness could act as a
preformed SEI reducing regeneration of SEI and lithium-ion
consumption during cycling. Less regeneration of SEI would
produce less SEI fragments, therefore resulting in its best rate
performance. Although G05A shows similar charge behavior at
high current density as G05U does, its cycling stability
deteriorated quickly at relatively high current density of 120
mA/g. The cycling stability of G0 deteriorated dramatically at
high current density, revealing its instable SEI causing low CE.
In addition, G5A exhibited the lowest specific charge capacity,
which can be probably due to its thick and unsmooth coating
with cracks (Figure S2). Despite the cracks, thick coating
provides an insufficient number of transport channels for
lithium ions, resulting in low specific capacity at both low-rate
and relatively high-rate cycling.
EIS was used to better understand how the cycling stability
and high-rate performance of the Al2O3-coated NG were
improved greatly. Figure 9 shows the Nyquist plots of the
samples in the fifth cycle at 0.01, 0.1, and 0.5 V, respectively.
The detailed data from EIS experiments are summarized in
a
Rct,* Ω
sample
0.01 V
0.1 V
0.5 V
0.01 V
G0
G05A
G05U
G005A
G005U
107
53
47
42
38
21
27
34
17
20
21
27
34
17
20
108
Symbols “#” and “*” reference the indicators given in Figure 9.
shown in Figure 7. Therefore, EIS measurement at ∼0.1 V is of
great significance. The high-frequency semicircle region is
related to lithium-ion diffusion in the SEI and coating layer (Rf,
#); the middle-frequency semicircle region is related to the
charge transfer resistance (Rct, *) between the active material
and the surface film; the low-frequency slope region represents
lithium-ion diffusion in the bulk material.15,33−36 As Zhang et
al. pointed out for anodes the thickness of SEI will increase and
decrease while discharging (lithiation) and charging (delithiation), respectively;32 Rf at low potential would be larger than
that at high potential accordingly.
The Rf of G0 was the highest (∼107 Ω) at 0.01 V while
almost the lowest (∼21 Ω) at 0.5 V, indicating its highly
variable thickness of SEI, as shown in Table 3. In contrast, the
Rf of G05U stayed at around 40 Ω (∼47 to ∼34 Ω) in the
range of 0.01 to 0.5 V, suggesting that its Al2O3 coating
provides a relatively stable SEI, preventing generation and
decomposition of SEI during cycling. Due to incomplete
reversibility of SEI regeneration,7 more thickness variation of
SEI would generate more fragments of SEI and consume more
lithium ions, which would increase the resistance and result in
degradation of cycling stability and high-rate performance
finally.
6517
DOI: 10.1021/acsami.6b00231
ACS Appl. Mater. Interfaces 2016, 8, 6512−6519
Research Article
ACS Applied Materials & Interfaces
G005A and G005U possessed relatively low Rf, but their Rct
was high especially at 0.01 V. This could be attributed to its
thinner coating that might not be insulating and strong enough
to act as a proper preformed SEI, so that SEI regeneration
would still happen and therefore lead to high Rct. At low current
density, low ohmic polarization of thinner coating could
support relatively high first cycle CE and specific capacity. At
higher current density, however, it displayed relatively low
specific capacity and deterioration of cycling stability probably
because of required regeneration of SEI.32 This explanation
agrees well with cycling performance in Table 1 and Figure 8.
Consequently, it implies that a smooth Al2O3 layer with a
proper thickness could act as a preformed SEI, reducing
regeneration of SEI and associated lithium-ion consumption
and increasing cycling stability especially at high current
density.
Shaanxi Province (Grant Nos. 2014JQ2079, 2014JM6231, and
2014JQ2-2007), the Fundamental Research Funds for the
Central Universities of China (Grant No. xjj2014044), and the
Deutscher Akademischer Austausch Dienst (German Academic
Exchange Service, DAAD).
■
4. CONCLUSIONS
A low-cost treatment to modify NG by coating with Al2O3
based on sol−gel method was demonstrated to improve its CE
(especially the CE in first cycle), cycling stability, and high-rate
performance. A smooth Al2O3 coating layer with proper
thickness can act as a preformed SEI reducing regeneration
of SEI and lithium-ion consumption during cycling. Advantages
of Al2O3 coating were theoretically examined by inspecting
relative energy levels in Li-ion batteries based on first-principles
DFT calculations. Due to its suitable bandgap and favorable
lithium-ions conduction ability,17,20,26 the Al2O3 coating
performs the functions as an SEI, preventing electron transfer
and allowing lithium-ion transport. Therefore, as a preformed
SEI, the Al2O3 coating reduces extra lithium-ion consumption
in commercial batteries. Possible future research topics can be
extended to the application for other anode materials and
further confirmation for the SEI change on the Al2O3 coating
layer by in situ Fourier transform infrared attenuated total
reflection spectroscopy or Raman spectroscopy.
■
ASSOCIATED CONTENT
* Supporting Information
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsami.6b00231.
Details of work functions and bandgaps for lithium,
Al2O3, and graphite, EIE, LUMO−HOMO gap, and
ΔEredof electrolyte solvents (PDF)
■
REFERENCES
(1) Verma, P.; Maire, P.; Novák, P. A Review of the Features and
Analyses of the Solid Electrolyte Interphase in Li-Ion Batteries.
Electrochim. Acta 2010, 55 (22), 6332−6341.
(2) Peled, E. The Electrochemical Behavior of Alkali and Alkaline
Earth Metals in Nonaqueous Battery Systems - The Solid Electrolyte
Interphase Model. J. Electrochem. Soc. 1979, 126 (12), 2047−2051.
(3) Aurbach, D. Review of Selected Electrode−Solution Interactions
Which Determine the Performance of Li and Li Ion Batteries. J. Power
Sources 2000, 89 (2), 206−218.
(4) Edström, K.; Herstedt, M.; Abraham, D. P. A New Look at the
Solid Electrolyte Interphase on Graphite Anodes in Li-Ion Batteries. J.
Power Sources 2006, 153 (2), 380−384.
(5) Xu, K.; von Cresce, A. Interfacing Electrolytes with Electrodes in
Li Ion Batteries. J. Mater. Chem. 2011, 21 (27), 9849−9864.
(6) Aurbach, D.; Markovsky, B.; Shechter, A.; Ein-Eli, Y.; Cohen, H.
A Comparative Study of Synthetic Graphite and Li Electrodes in
Electrolyte Solutions Based on Ethylene Carbonate-Dimethyl
Carbonate Mixtures. J. Electrochem. Soc. 1996, 143 (12), 3809−3820.
(7) Goodenough, J. B.; Park, K.-S. The Li-Ion Rechargeable Battery:
A Perspective. J. Am. Chem. Soc. 2013, 135 (4), 1167−1176.
(8) Wang, H.; Abe, T.; Maruyama, S.; Iriyama, Y.; Ogumi, Z.;
Yoshikawa, K. Graphitized Carbon Nanobeads with an Onion Texture
as a Lithium-Ion Battery Negative Electrode for High-Rate Use. Adv.
Mater. 2005, 17 (23), 2857−2860.
(9) Wu, Y. P.; Jiang, C.; Wan, C.; Holze, R. Modified Natural
Graphite as Anode Material for Lithium Ion Batteries. J. Power Sources
2002, 111 (2), 329−334.
(10) He, Y.; Yu, X.; Wang, Y.; Li, H.; Huang, X. Alumina-Coated
Patterned Amorphous Silicon as the Anode for a Lithium-Ion Battery
with High Coulombic Efficiency. Adv. Mater. 2011, 23 (42), 4938−
4941.
(11) Kim, S.-S.; Kadoma, Y.; Ikuta, H.; Uchimoto, Y.; Wakihara, M.
Electrochemical Performance of Natural Graphite by Surface
Modification Using Aluminum. Electrochem. Solid-State Lett. 2001, 4
(8), A109.
(12) Park, S. B.; Shin, H. C.; Lee, W.-G.; Cho, W. I.; Jang, H.
Improvement of Capacity Fading Resistance of LiMn2O4 by
Amphoteric Oxides. J. Power Sources 2008, 180 (1), 597−601.
(13) Riley, L. A.; Cavanagh, A. S.; George, S. M.; Jung, Y. S.; Yan, Y.;
Lee, S.-H.; Dillon, A. C. Conformal Surface Coatings to Enable High
Volume Expansion Li-Ion Anode Materials. ChemPhysChem 2010, 11
(10), 2124−2130.
(14) Jung, Y. S.; Cavanagh, A. S.; Riley, L. A.; Kang, S.-H.; Dillon, A.
C.; Groner, M. D.; George, S. M.; Lee, S.-H. Ultrathin Direct Atomic
Layer Deposition on Composite Electrodes for Highly Durable and
Safe Li-Ion Batteries. Adv. Mater. 2010, 22 (19), 2172−2176.
(15) Jung, Y. S.; Cavanagh, A. S.; Dillon, A. C.; Groner, M. D.;
George, S. M.; Lee, S.-H. Enhanced Stability of LiCoO2 Cathodes in
Lithium-Ion Batteries Using Surface Modification by Atomic Layer
Deposition. J. Electrochem. Soc. 2010, 157 (1), A75−A81.
(16) Kim, J.-S.; Johnson, C. S.; Vaughey, J. T.; Hackney, S. A.; Walz,
K. A.; Zeltner, W. A.; Anderson, M. A.; Thackeray, M. M. The
Electrochemical Stability of Spinel Electrodes Coated with ZrO2,
Al2O3, and SiO2 from Colloidal Suspensions. J. Electrochem. Soc. 2004,
151 (10), A1755−A1761.
(17) Jung, S. C.; Han, Y.-K. How Do Li Atoms Pass through the
Al2O3 Coating Layer During Lithiation in Li-Ion Batteries? J. Phys.
Chem. Lett. 2013, 4 (16), 2681−2685.
(18) Li, C.; Zhang, H. P.; Fu, L. J.; Liu, H.; Wu, Y. P.; Rahm, E.;
Holze, R.; Wu, H. Q. Cathode Materials Modified by Surface Coating
for Lithium Ion Batteries. Electrochim. Acta 2006, 51 (19), 3872−3883.
AUTHOR INFORMATION
Corresponding Authors
*(Y.X.) E-mail: [email protected].
*(R.H.) E-mail: [email protected].
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
The SEM and TEM work was done at the International Center
for Dielectric Research (ICDR), Xi’an Jiaotong University,
Xi’an, China; we also thank Ms. Dai and Dr. Lu Lu and for their
help in using SEM and TEM. We acknowledge Professor Xiang
Zhao in Xi’an Jiaotong University for his support with use of
the calculation code. This project is financially supported by the
National 111 Project of China (Grant B14040), the National
Natural Science Foundation of China (Grant Nos. 21203145,
21343011, and 21503158), the Natural Science Foundation of
6518
DOI: 10.1021/acsami.6b00231
ACS Appl. Mater. Interfaces 2016, 8, 6512−6519
Research Article
ACS Applied Materials & Interfaces
(19) Fu, L. J.; Liu, H.; Li, C.; Wu, Y. P.; Rahm, E.; Holze, R.; Wu, H.
Q. Surface Modifications of Electrode Materials for Lithium Ion
Batteries. Solid State Sci. 2006, 8 (2), 113−128.
(20) Hao, S.; Wolverton, C. Lithium Transport in Amorphous Al2O3
and AlF3 for Discovery of Battery Coatings. J. Phys. Chem. C 2013, 117
(16), 8009−8013.
(21) Delley, B. An All-Electron Numerical-Method for Solving the
Local Density Functional for Polyatomic-Molecules. J. Chem. Phys.
1990, 92 (1), 508−517.
(22) Delley, B. From Molecules to Solids with the Dmol3 Approach.
J. Chem. Phys. 2000, 113 (18), 7756−7764.
(23) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient
Approximation Made Simple. Phys. Rev. Lett. 1996, 77 (18), 3865−
3868.
(24) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.;
Pederson, M. R.; Singh, D. J.; Fiolhais, C. Atoms, Molecules, Solids,
and Surfaces: Applications of the Generalized Gradient Approximation
for Exchange and Correlation. Phys. Rev. B: Condens. Matter Mater.
Phys. 1992, 46 (11), 6671−6687.
(25) Tkatchenko, A.; Scheffler, M. Accurate Molecular Van Der
Waals Interactions from Ground-State Electron Density and FreeAtom Reference Data. Phys. Rev. Lett. 2009, 102 (7), 073005.
(26) Park, J. S.; Meng, X. B.; Elam, J. W.; Hao, S. Q.; Wolverton, C.;
Kim, C.; Cabana, J. Ultrathin Lithium-Ion Conducting Coatings for
Increased Interfacial Stability in High Voltage Lithium-Ion Batteries.
Chem. Mater. 2014, 26 (10), 3128−3134.
(27) Cui, X.; Green, M. A.; Blower, P. J.; Zhou, D.; Yan, Y.; Zhang,
W.; Djanashvili, K.; Mathe, D.; Veres, D. S.; Szigeti, K. Al(OH)3
Facilitated Synthesis of Water-Soluble, Magnetic, Radiolabelled and
Fluorescent Hydroxyapatite Nanoparticles. Chem. Commun. 2015, 51
(45), 9332−9335.
(28) Rosenqvist, J.; Persson, P.; Sjöberg, S. Protonation and Charging
of Nanosized Gibbsite (A-Al(OH)3) Particles in Aqueous Suspension.
Langmuir 2002, 18 (12), 4598−4604.
(29) Lichty, P.; Wirz, M.; Kreider, P.; Kilbury, O.; Dinair, D.; King,
D.; Steinfeld, A.; Weimer, A. W. Surface Modification of Graphite
Particles Coated by Atomic Layer Deposition and Advances in
Ceramic Composites. Int. J. Appl. Ceram. Technol. 2013, 10 (2), 257−
265.
(30) Paruchuri, V. K.; Nguyen, A. V.; Miller, J. D. Zeta-Potentials of
Self-Assembled Surface Micelles of Ionic Surfactants Adsorbed at
Hydrophobic Graphite Surfaces. Colloids Surf., A 2004, 250 (1−3),
519−526.
(31) Winter, M.; Besenhard, J. O.; Spahr, M. E.; Novák, P. Insertion
Electrode Materials for Rechargeable Lithium Batteries. Adv. Mater.
1998, 10 (10), 725−763.
(32) Zhang, J.; Wang, R.; Yang, X.; Lu, W.; Wu, X.; Wang, X.; Li, H.;
Chen, L. Direct Observation of Inhomogeneous Solid Electrolyte
Interphase on Mno Anode with Atomic Force Microscopy and
Spectroscopy. Nano Lett. 2012, 12 (4), 2153−2157.
(33) Shaju, K. M.; Subba Rao, G. V.; Chowdari, B. V. R. Li-Ion
Kinetics and Polarization Effect on the Electrochemical Performance
of Li(Ni1/2Mn1/2)O2. Electrochim. Acta 2004, 49 (9−10), 1565−1576.
(34) Funabiki, A.; Inaba, M.; Ogumi, Z.; Yuasa, S. i.; Otsuji, J.;
Tasaka, A. Impedance Study on the Electrochemical Lithium
Intercalation into Natural Graphite Powder. J. Electrochem. Soc.
1998, 145 (1), 172−178.
(35) Zhang, S. S.; Xu, K.; Jow, T. R. Electrochemical Impedance
Study on the Low Temperature of Li-Ion Batteries. Electrochim. Acta
2004, 49 (7), 1057−1061.
(36) Wu, X.; Wang, S.; Lin, X.; Zhong, G.; Gong, Z.; Yang, Y.
Promoting Long-Term Cycling Performance of High-Voltage
Li2CoPO4F by the Stabilization of Electrode/Electrolyte Interface. J.
Mater. Chem. A 2014, 2 (4), 1006−1013.
6519
DOI: 10.1021/acsami.6b00231
ACS Appl. Mater. Interfaces 2016, 8, 6512−6519